The invention relates, in general, to vacuum evaporators and, in particular, to a new and useful effusion-type evaporator cell utilizing a mechanical slide for controlling a vapor escape evaporator to adjust the vapor effusion rate quickly and accurately.
Vapor sources are known which are equipped with a so-called total shutter, i.e. with movable screens which are arranged over the opening of the vapor source and with which the vapor source can be opened or shut rather abruptly. Before this, so-called vaporization dishes were employed almost exclusively. What was appreciated in such vaporization dishes was the large surface area which permitted large vaporization rates. Some substances, however, tend to spatter as they are fused in the vaporization dish. The spattered material may be intercepted by one or more covers positioned above the vaporization dish and provided in a suitable manner with aperatures (French Pat. No. 1,451,345). Further known are vapor sources designed as closed vessels and equipped with nozzles having an adjustable orifice (German Pat. No. 742,257).
At a growing rate, vapor sources are being employed which are designed and operate as basically described in the article "The Thermal Decomposition of Sodium Carbonate by the Effusion Method", by K. Motzfeldt in J. Phys. Chem., Volume 59, pages 139 to 147, and the article "A High Temperature High Purity Source for Metal Beam Epitaxy", by R. F. D. Farrow and G. M. Williams in Thin Solid Films, Volume 53, (1978), pages 303 to 315.
Features which distinguish evaporative from so-called effusion cells are a small effective evaporative surface and a homogeneous temperature profile of the substance to be vaporized. The crucibles of effusion cells may be provided at their upper rim with covers having a more or less large central hole. In this way, the vapor effusion rate may be preselected. Vapor sources thus modified are known from data sheet No. 10,005,278 by the company Vacuum Generators. By heating the cover having the central hole, the molecular conditions of the evaporating substance may be controlled in addition. This is described in the article "The Effect of Arsenic Vapor Species on Electrical and Optical Properties of Gas Grown by Molecular Beam Epitaxy", by H. Kunzel, J. Knecht, H. Jung, K. Wunstel and K. Ploog, in Appl. Phys. A, Volume 28 (1982), pages 167 to 173.
It is further known from the article "Angular Distribution of Molecular Bemas from Modified Kundsen Cells for Molecular-Beam Epitaxy", by L. Y. L. Shen in Journal of Vacuum Science and Technology, Volume 15 (1978), pages 10 to 12, that the geometric distribution of the vapor beam can be controlled by means of crucible covers which are provided with a plurality of parallel bores.
By "effusion cells" within the context of this specification, evaporator cells are understood which are designed as closed vessels and have a vapor outlet whose size does not substantially affect the thermal equilibrium of the cell.
In molecular beam epitaxy (MBE) applications, a special effusion cell geometry has proved advisable which is described in the articles "On the Use of `Downwardlooking` Sources in MBE Systems", by D. M. Collins in Journal of Vacuum Science and Technology", Volume 20 (1982), pages 250 to 251, and "A Simple Source Cell Design for MBE", by R. A. Kubiak, P. Driscoll and E. H. C. Parke in Journal of Vacuum Science and Technology, Volume 20, (1982), pages 252 to 353.
In vapor sources of the vacuum deposition technique, it should be possible to adjust the vapor effusion rate to a value corresponding to the requirements of the coating process just to be, or being performed, and to keep it constant at that level. However, such an adjustment of the vapor effusion rate by adjusting the wattage supplied to the evaporated requires time constants of ten to sixty seconds.
Because of this thermal inertia of the evaporator, thermal fluctuations due to variations of the heating power and of ambient conditions (such as coolant circulation), cannot be compensated for quickly enough and, consequently, the vapor effusion rate cannot be kept constant with a satisfactory accuracy.
The drawback of the thermal inertia is disturbing especially in coating processes where substance mixtures are deposited, and the individual components of the mixture are evaporated from two or more evaporator cells. If, in addition, the composition of the layer is to be varied in the direction perpendicular to the surface, in accordance with a program, for example to produce so-called diffusion profiles of heterojunctions in semiconductor technology, the mixing ratio cannot be varied quickly enough.
It is true, as mentioned above, that arrangements permitting the adjustment of the orifice and then the vapor effusion from a nozzle mechanically, by means of a slide, have already been provided. Unfortunately, experience has shown that simple slides easily become sticky by the condensed evaporated substance, and thus lock. Furthermore, especially with small vapor holes, the problem arises that, due to condensate deposits on the edges of the slide, the aperture varies in an unpredictable way, which strongly affects the desired control characteristics.